One interesting development to come out of the particle
physics-cosmology interface is the role of de-Sitter space in
M-theory. If inflationary theory is correct, then the universe
must go through an accelerating (perhaps quasi-de-Sitter) phase at
early times. However, the successes of the standard cosmology (not
to mention our presence in the universe today) imply that this was
a transient phase. However, the recent observations of type IA
supernovae
[Perlmutter et al.(1999),
Riess et
al.(1998b)]
point to a second accelerating epoch, beginning at the
present time. Taking this data at face value, there are two
interesting possibilities: the acceleration could be caused by a
small non-zero cosmological constant, or by some type of energy
that redshifts sufficiently slowly as to cause acceleration, but
that will eventually cease to act. Let us focus on the former
possibility. If there exists a true cosmological constant in the
universe, then the late-time space-time will approach de-Sitter
space. This would seem to imply that de-Sitter space was a vacuum
of the underlying theory. In the context of string theory this may
be a problematic conclusion. Several authors
[Hellerman
et al.(2001),
Fischler et
al.(2001)]
have recently pointed
out that de-Sitter space seems to be incompatible with string
theory, at least at the level of perturbation theory. Other arguments,
based on upper bound on entropy in de-Sitter space
[Banks(2000)]
also challenge the viablity of string
theory (or any theory with infinite degrees fo freedom) in a universe
with a real cosmological constant.
If, as
measurements of the equation of state of the dark energy are
refined, and string (or M) theory matures and its non-perturbative
structure is understood, this tension remains, this may be a a way
for cosmology to constrain our fundamental theories.